In general, an instant messenger refers to software or hardware which enables two or more users to transmit a small amount of text data in real time. The instant messenger enables a user to register a talker and exchange a message with a talker in a talker list in a messenger screen. Recently, it became possible to use the messenger in a mobile communication terminal, as well as a Personal Computer (PC), and the use is getting rapidly increased.
With the increase of use of a mobile communication terminal, such as, a smart phone, user concern about personal information protection has been increased. Specially, as it became public that the instant messenger stores information on a personal conversation in a messenger sever, a concern about personal information leakage is getting increased. In this regard, major instant messenger providers are trying to prevent leakage of the personal conversation by encrypting and storing the personal conversation in the server such that a hacker cannot hack the information on the personal conversional from the server. However, there still remain a problem that the personal conversation could be leaked by sniffing in a network section between a personal terminal and a messenger server or by an ill-intentioned sever manager.